The basic process of nanowire formation on substrates by the VLS (vapour-liquid-solid) mechanism is well known. A particle of a catalytic material, (e.g., gold) on a substrate is heated in the presence of certain gases to form a melt. A pillar forms under the melt and the melt rises up on top of the pillar. The result is a nanowire of a desired material with the solidified particle melt positioned on top. While nanowire formation is well known, precise control of nanowire growth locations, growth directions, and growth alignment into lattices is not.
In VLS semiconductor nanowire growth, the positions of seed nanocrystals may determine the location of nanowire development. Typically, the positioning of nanocrystals has not been controlled to a desired degree of precision and/or uniformity. This has lead to nanowires developing at random and/or unacceptable locations on the substrate. These random patterns limit nanowire applications in photonics, sensors, electronics, and so on.
Conventionally, an additional nanopatterning step may have been added before nanocrystal nucleation in an attempt to control the nanocrystal position. For example, a silicon oxide mask with regular openings may be generated on top of a substrate to attempt to contain nanocrystals within the openings. However, the nanocrystal positions may still not be precisely controllable because the mask opening size is usually larger than the nanocrystal size. Thus, the nanocrystals may shift their positions within the openings. Additionally, the single crystal substrate surface may be contaminated during the mask patterning step and by the mask itself. This will interfere with the epitaxial growth of nanowires and may lead to nanowire growth in uncontrolled directions. These misaligned nanowires have limited value if any in applications like photonics, sensors, and electronics for example.